This invention relates to a rotor for a motor such as a permanent magnet type synchronous motor (PM motor) for use in a mobile body such as an electric car.
Heretofore, a system has been known as this kind of permanent magnet type synchronous motor, in which the rotor has magnetic saliency so that reluctance torque is utilized effectively, and it is categorized as an embedded magnet type or a surface magnet type, depending on the structure of the rotor.
The embedded magnet type has a structure of permanent magnets being embedded in the rotor, and constitutes a permanent magnet type synchronous motor utilizing reluctance torque.
On the other hand, the surface magnet type has a construction in which a rotor 1 is arranged, for example, as shown in FIG. 10. The rotor 1 comprises a core 2 made of a ferromagnetic material, having permanent magnets 3 bonded in the outer surface at a given intervals, and provided with projections 4 between permanent magnets 3. The surface magnet type is a permanent magnet type synchronous motor utilizing reluctance torque generated by the projections 4 of the rotor 1.
However, since in the embedded magnet type, permanent magnets are embedded in the core of the rotor, short circuit of magnetic flux of the permanent magnets happens inside the core.
On the other hand, since in the surface magnet type, short circuit of magnetic flux hardly happens and magnetic flux of the permanent magnets can be utilized effectively, the amount of permanent magnet used can be reduced.
However, since in the surface magnet type, the induced voltage waveform of the winding on a stator facing the magnets contains a lot of higher harmonics, torque variation (torque ripple) of the motor becomes larger, resulting in vibration of the rotor, and noise from the vibration.
In view of the foregoing, a first object of this invention is to provide a rotor for a motor capable of effecting improved torque variation due to improvement in the induced voltage waveform and of effecting improved magnet torque in a permanent magnet type synchronous motor utilizing reluctance torque.
A second object of this invention is to provide a rotor for a motor capable of effecting improved power factor of a permanent magnet type synchronous motor utilizing reluctance torque.
In order to achieve the first object of this invention, a first aspect of the invention is arranged as follows.
The first aspect is characterized by a rotor for use in matched relation with a stator provided in a permanent magnet type synchronous motor, wherein the rotor comprises a core made of a ferromagnetic material. A given number of first permanent magnets are mounted in the outer surface of the core circumferentially at equal intervals, and the first permanent magnets are arranged such that polarities N, S of the permanent magnets on the side facing the stator are disposed alternately. In the core between the center axis of the core and the first permanent magnets are embedded second permanent magnets corresponding to the first permanent magnets, respectively, and the arrangement of the second permanent magnets is the same as that of the corresponding first permanent magnets.
As described above, in the invention of claim 1, permanent magnets forming the rotor are arranged such that first permanent magnets provided in the outer surface of the core are separated from second permanent magnets embedded in the core.
Therefore, according to the first aspect, if the same total amount of permanent magnet as in a conventional surface magnet type rotor is used, the magnetic flux density generated between the rotor and stator is improved as well as the induced voltage waveform, so that torque variation of the motor can be improved, and magnet torque is also improved due to increased armature interlinkage magnetic flux.
In addition, in order to achieve the second object of this invention, the second and third aspects are arranged as follows.
The second aspect is characterized by a rotor for a motor according to the first aspect, wherein positions of the embedded second permanent magnets are offset from the mounting positions of the first permanent magnets by a predetermined angle in the direction of rotation of the core.
Also, the third aspect is characterized by a rotor for use in matched relation with a stator provided in a permanent magnet type synchronous motor, wherein the rotor comprises a core made of a ferromagnetic material. A given number of permanent magnets are mounted in the outer surface of the core circumferentially at equal intervals, and the permanent magnets are arranged such that polarities N, S of the permanent magnets on the side facing the stator are disposed alternately. The core is formed with projections between the permanent magnets mounted in the outer surface, and each projection has a cutout at the forward end in the direction of rotation of the core.
Features of the second and third aspects of the foregoing construction will be described below.
Since reluctance torque is utilized in the the first aspect or a conventional rotor shown in FIG. 10, the armature magnetic flux will increase. Thus, the power factor of the motor will be decreased. To improve this power factor, it is necessary to suppress the armature magnetic flux, but this will decrease the torque.
To eliminate the foregoing disadvantage, the construction set forth in the second and third aspects is adopted.
That is, in the second aspect, positions of the embedded second permanent magnets are offset from the mounting positions of the first permanent magnets by a given angle in the direction of rotation of the core. Further, in the third aspect, each projection has a cutout at the forward end in the direction of rotation of the core.
In the foregoing construction, since not only little change in magnet torque is effected, but the peak phase (angle of lead) can also be decreased in spite of a slight drop in reluctance torque, the peak value of the resultant torque (magnet torque plus reluctance torque) is hardly lowered.
Further, since in addition to a drop in armature magnetic flux, its phase also changes, the phase difference between the induced voltage and current is decreased, improving the power factor of the motor. Therefore, the maximum output of the motor can be increased, which is effective especially to lower-voltage applications using a battery as a power source.